WASHINGTON (Reuters) - President Donald Trump's nominee to head the U.S. Immigration and Customs Enforcement (ICE) is retiring, the Department of Homeland Security said on Monday, days after Democrats had questioned why the agency still did not have a Senate-confirmed head.

The 57-year-old Thomas Homan had planned to retire in January 2017, but agreed to stay on to serve as acting director until the Trump administration found a permanent head.

He initially agreed to accept Trump's nomination in November to take that role himself, but earlier this year told the department he wanted to retire mid-2018, a department official said.

Homan was previously executive director of the ICE's enforcement and removal operations, leading its efforts to arrest and remove undocumented immigrants since 2013.

"The decision to leave federal service after more than 34 years is bittersweet, but my family has sacrificed a lot in order for me to serve and it's time for me to focus on them," Homan said in a statement. He praised the 20,000 ICE employees "who serve this agency and protect our nation, increasingly in the face of unfair and false criticism from politicians and the media."

The Trump administration's aggressive immigration enforcement has faced heavy criticism from immigration advocates. Trump made tougher enforcement a centerpiece of his campaign, along with a pledge to build a wall at the U.S.-Mexican border.

On Friday, 17 Democratic senators questioned why Homeland Security had not responded to questions from Congress about Homan's nomination for more than six months and questioned some of his statements.

"The absence of a Senate-confirmed head of ICE for more than a year hinders congressional oversight and the efficient operation of the agency and is troubling in any circumstance," they wrote.

Democrats said ICE had greatly reduced the use of discretion and sharply increased arrests and detentions of immigrants with no criminal background.

House Judiciary Committee Chairman Bob Goodlatte, a Republican who was deeply critical of what he believed were weaker immigration controls under former President Barack Obama, praised Homan for "helping to steer ICE back to its core mission of immigration enforcement and public safety."

The American Civil Liberties Union has filed lawsuits against the government, seeking to challenge actions that it says are separating families.

In January, ICE announced a new set of guidelines in response to concerns that immigration agents were picking up people targeted for deportation after court appearances on other matters.